There are 500,000 more females than males in Hong Kong, but that's on course to change because for several years more boys than girls have been born in the city, researchers have found.
The researchers, working in Queen Elizabeth Hospital's obstetrics department, recorded 114 male births for every 100 girls born in 2010.
Since 2005 the imbalance has been exaggerated by the influx of mainland women to give birth, as they are more likely to deliver sons than daughters.
"[The unbalanced sex ratio at birth] will result, after a few decades, in a large excess of men," the report said. "Consequently, many men will be deprived of marriage and parenting, and thus [there will be] adverse effects on the well-being of individuals, societal stability and security."
However, it will take decades for a scarcity of women to manifest itself. The current excess of women over men helps to explain why, in 2011, 98,700 women aged 30 to 34 were unmarried, according to the Census and Statistics Department (CSD).
The research, published in the Hong Kong Medical Journal today, covers more than 54,000 babies born in the public hospital in Jordan on the Kowloon peninsula. It found the sex ratio at birth rose from 106.6 boys for every 100 girls born between 2001 to 2005, to 111.4 between 2006 and 2010.
The ratio is considered high when it gets above 107.
Mainland couples, many of whom are allowed only one child, have a strong preference for sons over daughters. Women are more likely to abort a female fetus during pregnancy and be more willing to spend money on having a baby in Hong Kong if they know they are carrying a boy, the report said.
The research was completed before Chief Executive Leung Chun-ying announced a "zero-birth quota" for mainland women after complaints hospitals were struggling to cope with their influx and they were denying local women maternity beds. The measure has greatly reduced births to mainland mothers.
While that could mean the sex mismatch at birth will not remain as high as it was between 2006 and 2010, CSD data shows more boys than girls have been born for the past 20 years.
The researchers urged the government to educate the public about gender equality, and address the gender gap.